1. Field of the Invention
This invention relates to coal liquefaction and is particularly concerned with integrated liquefaction processes in which coal liquids produced by the treatment of feed coal with molecular hydrogen and a hydrogen-donor solvent are subsequently hydrogenated for the production of recycle solvent and, optionally, additional liquid products.
2. Description of the Prior Art
Among the more promising processes for the production of liquid hydrocarbons from coal are those in which the feed coal is first contacted with molecular hydrogen and a hydrogen-donor solvent in a liquefaction zone at elevated temperature and pressure and a portion of the liquid product is then catalytically hydrogenated in a solvent hydrogenation zone to generate solvent for recycle to the liquefaction step and produce additional liquid products. Hydrogenation of the liquid in the solvent boiling range is generally carried out at a pressure similar to or somewhat lower than that employed in the liquefaction zone and at a somewhat lower temperature. To supply the heat required to raise the solvent boiling range liquid to the hydrogenation temperature, it has been proposed that all of the vaporous product taken overhead from the liquefaction zone be passed directly to the solvent hydrogenation zone without cooling and that the quantity of coal liquids are recycle hydrogen which is mixed with the vaporous product and fed to the hydrogenation zone be adjusted so that the combined feed stream is maintained at the required hydrogenation temperature. This eliminates the need for a furnace to preheat the feed stream. Because the hydrogenation reaction is exothermic, additional cold feed is introduced into the hydrogenation zone downstream of the initial inlet point to quench the reaction and at the same time heat this additional feed to the necessary hydrogenation temperature.
Although the process described above has advantages over earlier processes from the standpoint of conserving thermal energy, it poses certain operational problems which tend, at least in part, to offset the heat conservation advantages. The use of the liquefaction vapors to provide all of the heat needed to raise the initial increment of the liquid feed to the hydrogenation temperature and thus eliminate the need for a furnace limits the ratio in which liquid and vapor can be introduced into the initial stage of the hydrogenation zone and imposes restrictions with respect to the hydrogen partial pressure in the initial stage. In addition, the cold feed introduced downstream of the initial stage has a relatively short residence time within the hydrogenation zone and hence uniform hydrogenation to achieve maximum solvent and product yields may be difficult to obtain. Overhydrogenation may sometimes occur. Moreover, the introduction of relatively cold feed into the reaction zone at one or more points downstream of the initial inlet makes effective contacting of the feed and hydrogen more difficult to achieve, may promote product degradation and the production of excessive quantitues of gas and low molecular weight hydrocarbons, and makes the overall reaction difficult to control. As a result of these and related disadvantages, the overall efficiency of such a process may leave much to be desired.
More recently, and in an effort to avoid the operating problems associated with the process described above, it has been proposed to cool the gaseous product from the liquefaction reactor so as to recover additional liquids therefrom and to effectively operate the hydrogenation portion of the process separately from the liquefaction portion thereof. In this method of operation, the vapor from the liquefaction step is, generally, cooled to recover additional liquid therefrom and then combined with makeup hydrogen and used as a "treat-gas" in the hydrogenation step of the process. Also, in this method of operation, the feed to the hydrogenation zone is, generally, preheated and the hydrogenation reaction quenched by recycling cooled liquid and/or gas from the hydrogenation zone. For reasons believed readily apparent, this method of operation avoids the "operational" problems associated with the previously described process. The recycle of vapor and/or liquid from the hydrogenation zone, however, increases the equipment requirements since additional pumps and/or compressors, heat exchangers, etc., will be required. Also, especially when vapors are recycled, additional processing steps are required since hydrogenation catalyst poisons must be removed from the recycle stream. As a result, the initial investment and the continuing operating costs are increased when this particular method is used. Moreover, since the number of processing steps is increased, process control and operation is, generally, more complicated. The need, then, for a process offering the operational advantages associated with separate control of both the liquefaction and hydrogenation sections without the increased equipment requirements and processing steps is believed to be readily apparent.